(1) Field of the Invention
This invention relates to a battery balance circuit, and more particularly relates to a battery balance circuit which can balance battery voltages of battery cells when the battery cells are in a charge state.
(2) Description of the Prior Art
As portable electronic products are developed rapidly, the demand for rechargeable batteries becomes increasingly larger. Rechargeable batteries include a conventional Ni—Ca battery, a Ni—H battery, and a Li-ion battery as well as the recently developed Li-polymer battery. The voltage provided by a different type of rechargeable battery varies, and the required operating voltages of the portable electronic products are also different. Therefore, battery manufacturers usually connect several batteries in series to form a battery module for providing a voltage to meet the operating voltage requirement of portable electronic product.
However, the electric power storage capacities of batteries in a battery module are different, and the actual use of electric power storage capacity of the battery module is determined by the battery with the lowest electric power storage capacity. In addition to the variation of factory default electric power storage capacity of each battery in a battery module, a battery also self-discharges electric power when not being in use. Since each battery self-discharges electric power at a different rate, an unbalanced electric power storage capacity will occur among the batteries, and the usable capacity of the battery module is reduced as the operation time of the battery increases, and the operation efficiency of the battery module becomes lower, and the operation time of the battery module from fully charged to empty becomes shorter.
FIG. 1 is a schematic diagram of a conventional battery balance circuit. A battery module comprises two battery cells BAT1 and BAT2 connected in series. A charge circuit 40 is coupled to the battery module for providing a charge current Ich to charge the battery cells BAT1 and BAT2. A protection circuit 20 is coupled to the battery module, and determines states of the battery cells BAT1 and BAT2. The protection circuit 20 controls a charge/discharge switch 25 in response to the states of the battery cells BAT1 and BAT2. When any one of the battery cells BAT1 and BAT2 is fully charged, the protection circuit 20 generates a fully-charged protection signal CO to cut off the charge/discharge switch 25, so as to stop charging the battery cells BAT1 and BAT2. When any one of the battery cells BAT1 and BAT2 is over-discharged, the protection circuit 20 generates an over-discharged protection signal DO to cut off the charge/discharge switch 25, so as to stop discharging the battery cells BAT1 and BAT2. A charge judgment circuit 30 is coupled to the charge/discharge switch 25, and detects a voltage difference between two terminals of the charge/discharge switch 25. Accordingly, the charge judgment circuit 30 determines whether the battery module is in a charge state. If yes, the charge judgment circuit 30 generates a balance enable signal CBEN. A balance circuit 10 is coupled to the battery module, and is activated when receiving the balance enable signal CBEN. Then, the balance circuit 10 detects a voltage difference between the battery cells BAT1 and BAT2, and accordingly determines whether to execute a voltage balance process. If the balance circuit 10 determines the battery cells BAT1 and BAT2 in the battery module are in an unbalance state, the balance circuit 10 performs the voltage balance process to reduce the voltage difference between the battery cells BAT1 and BAT2.
FIG. 2 is a charge current waveform of a conventional battery. In general, the charge current is substantially decreased with time, regardless of the charging mode for the batter. The charge judgment circuit 30 determines whether the battery module is in a charge state according to a voltage drop across the charge/discharge switch 25 when the charge current Ich flows therethrough. Consequently; and when the charge current Ich is lower than a detectable threshold OPTH, the charge judgment circuit 30 determines that the battery module is not in a charge state and thus stops generating the balance enable signal CBEN. At this time, the balance circuit 10 also stops the voltage balance process. As shown in FIG. 2, a time period of the charge current Ich being higher than the detectable threshold OPTH is short. Therefore, the voltage balance process executed by the balance circuit 10 lasts a short period, and the balance effect is unobvious. Besides, the determination of the charge judgment circuit 30 is interfered by influences, such as noises and offset voltages thereof. Thus, the size of the detectable threshold OPTH has to be sufficient large against the influence, and so the detectable threshold OPTH is hard to be decreased.